Electron emitters, also known as field emitters, are well known in the microelectronics art. Such emitters are widely used to emit electrons into a vacuum or gaseous region in electron beam lithography tools, scanning tunnel microscopes, electron guns, field ionizers, vacuum integrated circuits, and other devices.
There are two general classes of emitters, known as "hot" and "cold" emitters. Typically, cold emitters are microelectronic structures with small protruding points. Quantum mechanical tunnelling causes the points to emit an electron beam upon application of an appropriate voltage thereto. A major problem for conventional cold emitters is the extremely large electric field needed to emit electrons from the device surface, leading to the need to concentrate the electric field. In order to concentrate the electric field, conventional cold emitters are formed with a pointed emission surface. These pointed microelectronic structures are difficult to fabricate. In particular, the fine geometries of emitter arrays and the delicate point structures result in a very complex and expensive fabrication process. In addition, because of the complexity of the process a high quality emitter point is difficult to achieve.
The second class of emitters, known as "hot" emitters, use a reverse-biased p-n junction to generate a high current density of electrons for emission, without the need for field concentrating microelectronic points. Here minority carriers are injected into the depletion region of the junction and are accelerated by the large electric field within the region. For the electron population to remain "hot", the distance over which they are accelerated should not be larger than the mean free path for electrons. Since the current injected into the junction is of the order of the reverse biased saturation current of a p-n junction, it is necessary to increase the magnitude of this current to achieve a reasonable current density in the vacuum. The only way to multiply the current is through impact ionization which thermalizes the electron population and hence decreases the efficiency of emission. Another scattering mechanism that substantially cools the injected hot electron distribution is plasmon-phonon scattering in the n+ region required to bias the p-n junction. The n+ thickness should be minimized to minimize electron scattering but has to be maximized to decrease input resistance and hence increase efficiency. These contrasting needs lead to inefficiency. So far, the maximum reported efficiency obtained using a reverse biased p-n junction emitter is about eight (8%) percent.
In conclusion, known emitters either require pointed emitter surfaces which are difficult to accurately fabricate, or require reverse biased p-n junctions which are highly inefficient. Accordingly, known emitters have been expensive, inefficient, or both.